doc: update function app naming rule (#16592)

see: 
1. Azure/azure-functions-host#2015
1. https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/resource-name-rules#microsoftweb

website/docs/r/function_app.html.markdown

@@ -191,7 +191,7 @@ resource "azurerm_function_app" "example" {
 
 The following arguments are supported:
 
-* `name` - (Required) Specifies the name of the Function App. Changing this forces a new resource to be created.
+* `name` - (Required) Specifies the name of the Function App. Changing this forces a new resource to be created. Limit the function name to 32 characters to avoid naming collisions. For more information about [Function App naming rule](https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/resource-name-rules#microsoftweb)
 
 * `resource_group_name` - (Required) The name of the resource group in which to create the Function App.